1. Field of the Invention
The present invention relates generally to the field of electronic imaging, and more particularly to inspection of specimens such as semiconductor wafers and photomasks using TDI (Time Delay Integration) sensors.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask. Certain advanced semiconductor defect inspection systems can detect defects on the order of 30 nm in size during a full inspection of a 300 mm diameter wafer. Such defects are seven orders of magnitude smaller than the wafer itself.
These types of optical systems may employ sophisticated sensors, including but not limited to TDI sensors. TDI sensors exhibit increased throughput for wafer inspection systems and photomask inspection systems over other types of sensors by more than one order of magnitude. FIG. 1 illustrates a typical TDI sensor. From FIG. 1, an array of pixels make up the imaging region 101. A current state-of-the-art TDI sensor according to FIG. 1 may contain a 256×2048 array or larger image area. In a typical arrangement, a lamp, laser beam, or other bright illumination source illuminates the semiconductor wafer surface. The wafer surface reflects light onto the TDI sensor, and at the points where light strikes the sensor the sensor may generate photoelectrons.
The TDI sensor typically scans a magnified image of the wafer. The sensor continuously accumulates charge as it scans the wafer, and the sensor transfers charge along a column of pixels 102 at generally the same rate at which the sensor moves with respect to the wafer image. In the orientation of FIG. 1, the sensor moves charge vertically from one pixel to the next.
TDI sensors typically contain channel stops 103, represented by the solid vertical lines in FIG. 1. These channel stops 103 prevent the movement of electrons or charge from one column to another within the imaging region 101. Electron movement is generally inhibited until the electrons reach the serial registers 104 at the edge of the sensor, where the serial registers are represented by gray rows of pixels.
When charge reaches the last pixel in a column, the charge moves to the serial register 104. The serial register 104 transfers the charge horizontally, pixel by pixel, until the charge reaches read-out stage and read-out amplifier or amplifiers 105. A transfer gate 106 or similar structure typically controls charge movement between the imaging region 101 and the serial register 104.
Certain TDI sensors have only one read-out amplifier 105, typically positioned at the end of the serial register 104. Other TDI sensors, such as the one shown in FIG. 1, have multiple read-out amplifiers 105 to decrease the time required to read the contents of the pixels in the serial register.
For several reasons, previous TDI sensors exhibit less than optimal functionality. Prior TDI sensors employ a method called “burst clocking,” whereby the TDI sensor may transfer a charge from pixel to pixel, where the graph of voltage changes sharply from positive to negative and back again. Previous TDI sensors employing burst clocking do not exhibit optimal speed in transferring the pixel charge, and tend to be highly sensitive to timing jitter. Such sensors can exhibit high levels of power dissipation and have a relatively low charge transfer efficiency. Further, previous TDI sensors tend to exhibit high dispersion of clock waveforms, low modulation transfer functions, and a higher probability of electromigration. Further, TDI sensors employing burst clocking generally do not perform well when environmental conditions or subtle operating changes occur.
When implementing a TDI sensing design, certain issues may arise, most notably feed through time, charge transference, timing jitter generated by on-board electronics, and an imperfect ground return path within a high speed sensor implementation, each introducing certain errors to the signals generated. Smooth operation is desirable in CCD or TDI sensing hardware implementations.
It would therefore be beneficial to provide an implementation of relatively smoothly operating sensor for use in conjunction with semiconductor wafer or photomask inspections that overcome the foregoing drawbacks present in previously known electronic imaging systems. Further, it would be beneficial to provide a sensing implementation and overall optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.